Azo Dye Decolorization by
Chemical, Physical and Biological Techniques
Dr. Mark Moskovitz and Gary
Witman, MD
The textile industry utilizes
more than 10,000 different dyes and pigments, with
the annual worldwide production nearing 2,000,000
tons. The azo class of synthetic dyes is by far the
most commonly used dye, representing more than 50%
of all dye production. There are more than 2000
structurally different azo dyes in commercial usage.
Many different techniques are being used in an
attempt to effectively and efficiently remove azo
dyes from the wastewater effluents of the textile
and other dye stuff industries, among which are the
leather, food, cosmetics, color photography,
pharmaceutical and paper product industries.
For both environmental and
health reasons it is essential to completely remove
these azo dyes before they reach the effluent stream
going out to the discharge water supply as both the
dyes and their intermediate degradation products are
mutagenic and may become carcinogenic in anaerobic
conditions. Significant amounts of these dyes are
lost during the dyeing process. Depending on the
class of dye, this loss in waste waters may range
from 2% of the original concentration for basic
dyes, to as high as 50% for reactive dyes, with an
overall average loss of 15%. The azo dye containing
effluent discolors the water and increases the
biochemical oxygen demand of the contaminated water,
(Refer to previous technical paper Utilization of
Specialized Activated Alumina for Decolorization by
Moskovitz and Witman) creating anoxic conditions
which may be lethal to aquatic species. The dye mass
concentration from most azo containing wastewater
discharge is in the range of 10-50 mg/liter of
effluent.
Azo dyes are characterized by
nitrogen to nitrogen double bonds (-N=N-). They
contain at least one and up to four azo groups
usually attached to two radicals of which at least
one but usually both are aromatic groups. The color
of azo dyes is determined by the azo bonds and their
associated chromophores and auxochromes. The
chromophore is a radical configuration consisting of
conjugated double bonds containing delocalized
electrons. An auxochrome is a functional group of
atoms with nonbonded electrons which when attached
to a chromophore, alters both the wavelength and
intensity of absorption, and as such is able to
increase the color of any organic compound. Azo
bonds are the most active bonds in azo dye
molecules, which may be broken down through
oxidation by hydroxyl radicals or reduced by
electrons. The breakdown in these azo bonds leads to
the subsequent decolorization of dyes.
However, because the goal in
coloring is to provide color permanence azo dyes are
manufactured to be resistant to biological attack,
light, heat and oxidation. As noted above, many
processes have been investigated to clear up the
color in textile wastewater. No one system has yet
proven itself on an industrial scale to provide a
satisfactory solution. Many physical and chemical
methods may be expensive and may not be effective.
The physical and chemical methods which have been
used for azo dye removal include adsorption,
coagulation and membrane processes. Membrane
technologies are very effective, but utilize
significant amounts of energy. Furthermore, the
biological processes so far developed have been
relatively ineffective.
Azo dyes are generally resistant
to biodegradation due to their complex structures.
Many different bacterial and fungal azo dye
degrading microorganisms have been tested in an
attempt to identify an affordable biological
solution for the decoloration of azo dyes which may
be accomplished within a time period of minutes to
hours. Degradation of azo dyes by bacteria is
obtained by bacterial azoreductase enzymes cleaving
azo bonds, followed by primarily subsequent
anaerobic degradation of resulting aromatic amines.
Responsible bacterial DNA fragments have been
isolated and azoreductase genes have been cloned.
The bottleneck of this sequence is the anaerobic
reduction, which is a slow process sped up through
the use of catalysts such as quinones as redox
mediators. The quinone redox mediators which are
currently being utilized are
1,2-naphthoquinone-4-sulfonate (NQS) and
anthraquionone-2,6-disulfonate (AQDS). The addition
of immobilized quinones increases up to 8 fold the
rate of decolorization, with these electron
mediators transfering reducing equivalents from an
electron co-substrate to the azo linkage.
Another biological approach
which is being looked at on a large scale basis is
the use of white rot fungi, with the non specific
extracellular lignin modifying enzymes (LME) of
these fungi capable of degrading many dyes. With
fungi, decolorization occurs by aerobic ligninolytic
degradation in association with lignin peroxidase.
Unfortunately the long growth cycle of fungi and the
moderate decolorization rate of fungi limit the
usefulness of this technique.
Advanced oxidation processes
have been addressed characterized by the production
of hydroxyl radicals (OH-) as the primary oxidant.
The Fenton reagent is an effective oxidizer, but it
produces a substantial amount of Fe(OH)3 precipitate
as well as additional water pollution caused by the
catalyst which has been added as a salt. Some
investigators have attempted to use zero valent iron
metal (Fe 0) but with limited success.
A superior system may lie in a
two step approach using an initial polishing step
with activated carbon, followed by the use of
specially activated decolorizing alumina. The use of
activated carbon lies in the ability to conduct
electrons as well as being a redox mediator
containing surface quinonic structures as well as
other functional groups including aromatic rings. A
high concentration of dye on the carbon surface
helps electron transport from the electron donor
acetate to the azo linkage since catalysis mostly
takes place in the adsorption layer.
As discussed in our earlier
paper the use of decolorizing activated alumina were
initially designed for the color removal of the
taxane compounds harvested from the bark and needles
of the Pacific Yew trees. Modifications were made to
the pore size, particle distribution and the pH of
specially designed alumina. Subsequently it was
determined that these modifications made for removal
of color from taxane compounds were also appropriate
for the removal of azo compounds.
The advantage of using activated
alumina to bind and decolorize azo dyes lies in the
amphoteric properties of alumina. Both acid and
basic dyes are able to bind on to the same particle.
This unique property of decolorizing alumina,
coupled with the ability to reactivate alumina at
temperatures in excess of 400 C and reuse the
alumina makes this the most cost efficient method
for azo dye extraction.
Additionally, the most effective
method for industrial scale azo dye removal may lie
in using a tandem reactor in which azo dye waste is
removed in a two step physical process. In the first
step activated carbon is used as a redox mediator
and scrubber. Polishing is then accomplished using
specialty activated alumina, with the effluent
discharge void of color. This two step physical
decolorization process is more cost efficient, safe
and reproducible than any other commercially
available processing method.
The advantages offered from
using adsorption methods implementing activated
alumina and activated carbon more than overcome any
theoretical advantages which potentially could arise
from use of bacterial or fungal based biological
systems.
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